Abstract: Human CD34+ hematopoietic precursor cells can readily be obtained from the peripheral blood of adults. Treatment of these cells with various cytokine cocktails can induce efficient production of cells differentiated along erythroid, megakaryocytic or myeloid lines. We find that different cytokine cocktails induce rapid and extensive patterns of change in gene expression that are different for different cytokine cocktails. The prominence of these changes suggests that they occur in a large fraction of the CD34+ cells and may modulate the patterns of differentiation of individual cells. The recent development of methods that permit global measurements of mRNA expression, and DNA methylation in single cells make it possible to investigate the role of cytokines in lineage promotion at a level of specificity hitherto unavailable. In the present application we propose to use a combination of single cell genomic scale analyses, and cell tagging and labeling approaches to establish if the same cell can respond differently to different cytokine treatments, and to investigate whether these different responses result in differentiation along alternate lineages. The ultimate goal is to determine the nature of early transcriptional responses to lineage specific cytokines and whether these are instructional as well as permissive for lineage specification.